In recent years, along with a tendency to high brightness and high efficiency of a light emitting diode (hereinafter sometimes referred to as LED) element, a light emitting device employing a LED element for a backlight of a mobile phone, a large sized liquid crystal TV, etc. or for general illumination, has been used. Therefore, members peripheral to the LED element are also required to have higher performance. Heretofore, as a substrate for a light emitting element on which a light emitting element such as a LED element is to be mounted, one comprising a resin material has been used. However, it is likely to be deteriorated by heat or light due to the high brightness tendency of a light emitting element, and use of one made of an inorganic material, for example, one made of a ceramic, has been studied.
A ceramic substrate may, for example, be an alumina substrate or an aluminum nitride substrate to be used for a wiring board. A ceramic substrate is promising as a substrate for a light emitting element since it has high durability to heat or light as compared with a resin substrate. However, a ceramic substrate has a low reflectance as compared with a resin substrate, and accordingly light from the light emitting element will leak to the back side of the substrate, whereby the light intensity on the surface side tends to be decreased. Further, as a ceramic substrate is usually hardly sintered, high temperature firing at higher than 1,500° C. is required, thus leading to an increase in the process cost.
In order to solve such problems, use of a low temperature co-fired ceramic (hereinafter referred to as LTCC) substrate started being studied. An LTCC substrate usually comprises a composite of glass and a ceramic filler of e.g. alumina, which can be sintered at a temperature of from about 850 to about 900° C. which is lower than that of a conventional ceramic substrate since it is sintered employing low temperature flowability of glass. Therefore, it can be co-fired with an Ag conductor to be a wiring conductor, and thus the cost can be reduced as compared with a conventional ceramic substrate. Further, a higher reflectance than that of a conventional ceramic substrate can be obtained, since light is diffusely reflected on an interface between glass and the ceramic filler. Further, as it comprises an inorganic substance, it has sufficient durability to heat and light.
As a ceramic filler to be incorporated in the LTCC substrate, typically, for example, an alumina powder has been known. Further, it has also been known to use a titania powder or a zirconia powder together with an alumina powder (for example, Patent Documents 1 and 2).